What’s going on with the left prop on my Piper Mojave?

Q: I just bought a Piper Mojave. It has 1,200 hours on both engines. Pre-buy, including oil analysis, was good.

During a maiden flight, the left prop feathered on prop check. We checked the dome pressure, it was low. We topped it to 41 psi.

Took off for an hour flight. Landed and left prop feathered on landing roll out. Checked the governor. No problem. Then we increased the engine oil pressure. Did some test flying, all OK.

The next day we flew for 30 minutes, landed and left prop feathered on landing roll out. Unfeathered it. Flew back, landed high RPM and extra power. No problem. Slow shut down with idling at low RPM 900 or so. The left engine was at 40 and right at 45. Engine did not feather.

So the increase in left engine oil pressure worked a bit, but not long enough. As long as the left engine pressure holds above 40 I guess it does not go into feather. The times it has gone to feather it was so spontaneous that we did not notice the oil pressure check.

Also during the feather check the left prop recovery is sluggish and drop is slightly more than the left.

Any suggestions ?

Rohit Singla, via email

A: Rohit, it looks like you may have an interesting situation here and I make no promises that I’ll be able to come up with the answer, but let me make a few suggestions.

I remember the Piper Mojave very well, having spent much time at the Piper Lock Haven factory during the time this aircraft was being developed and produced. As you may know, Piper only produced 50 of these aircraft and one of the first customers was The Royal Flying Doctors of Australia. To my knowledge, these aircraft served them well even while operating in some very hostile conditions.

Now let’s get back to your situation and see what I can come up with. I took note that there may have been less air pressure in the prop dome than was called for. Once it was serviced, it apparently improved the situation somewhat, but still not enough to prevent a repeat of the prop going into feather on landing roll out.

I’d recommend that before you invest any more money at this point, let’s try something simple.

Since you didn’t mention where this aircraft is being operated geographically, we may have a high ambient temperature condition that is contributing to our problem.

Another very important factor that may cause a problem is the true oil temperature, which you failed to mention also. While the maximum oil temperature for all Lycoming engines is 245° F, it would be interesting to know exactly what your oil temperature was at the time you experienced the prop going into feather.

You also didn’t mention what type of oil you are using in the engine, but I’d guess that it’s probably a multi-viscosity oil, probably like 15W50 or 20W50.

Because I suspect high temperature may be part of the equation here, I’d like you to switch to a single weight ashless dispersant oil. I’d suggest 50 weight or, if you are operating in a hot climate, I’d even consider 60 weight.

If you have any questions regarding which weight to use, I’d suggest you have your maintenance facility review the latest revision of Lycoming Service Instruction 1014.

My reasoning here is the hotter the engine oil temperature, typically we expect to see a lower oil pressure, which could result in lower oil pressure going to the prop dome. With insufficient oil pressure to the dome, the prop will go into the feather position.

I’m almost positive that the V2AD engines all had the adjustable oil pressure valve assemblies installed at the Lycoming factory, so I understand your maintenance facility has made adjustments to those to increase the oil pressure at idle RPM? Further adjustment may provide even better results.

If you check the Lycoming Parts Catalog for the V2AD engine, you may find that there are optional oil pressure relief valve springs available, some of which may have a longer free length or a larger wire diameter. It would surprise me if a change in that spring would be required, but I don’t want to overlook any possibility.

Another thing that comes to mind is the condition of the engine oil coolers. Not knowing the background of this aircraft, is there a possibility that the oil coolers are not receiving the proper amount of inlet air, resulting in less then required performance?

Any baffling discrepancies in that area could also cause issues.

While we’re talking about the engine oil coolers, I’d be certain that the thermostatic oil by-pass valve (vernatherm) is seating properly. Your maintenance facility may want to consider complying with Lycoming Service Instruction 1316A if the other recommendations mentioned above fail to conquer the problem.

The important thing to remember when complying with SI 1316A is to follow the instructions very carefully when recutting the seat. My recommendation is to always install a new vernatherm after recutting the seat.

With regard to the sluggish operation during the feather check and the left prop versus the right prop, I believe we can chalk that up to the lower oil pressure on the left engine.

Let’s hope by going to a heavier weight oil we can solve this problem, which would also be the least expensive way, and once outside ambient air temperatures go down, you’ll be able to return to using the oil that you are presently using.

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Comments

As I recall the dome air pressure assists the spring in making the prop feather. Low air pressure would slow the feathering action.
There are centrifugal stop pins that keep the prop from feathering on every shut.When an engine is deliberately feathered the blades turn past the stop point before the feather lockout pins engage.
If there are any weak seals, internal oil leaks at reduced power the spring and dome pressure will feather the prop.
If I were the operator I’d ground the airplane until the prop, particularly the hub and the governor were overhauled.

I’m just one opinion but, uncommanded feathering of a propeller is very bad. The propeller manufacturer has manuals with troubleshooting section. Hartzell Propeller Owners Manual covers the issue of: “Propeller Goes To Uncommanded High Pitch (or Feather)”.

No reason to go through each of the possibilities here in the reply section. By-the-way, none of the possible causes are something you want to be test flying the aircraft while trying to figure things out.

Might be a crack in the crankshaft. I saw a Beech Baron, prop fly off on the ramp. The pilot reported a uncommanded engine prop feathered in Flight. Maintenance found noting wrong. The pilot did a static run up, the prop went right though the maintenance hanger, without hitting anybody. The Baron had a gear up landing, several hundred hours ago. Engines were not torn down. Finding, the other engine also had a crack in the crankshaft. Don, 910 5287769